In the past, high purity phosphoric acid or alkali metal phosphates including, for example, trisodium phosphate, sodium tripolyphosphate, tetrasodium pyrophosphate and disodium phosphate were prepared by the thermal acid method described in Slack, Phosphoric Acid, V. l, pp. 927-966, Marcel Dekker, Inc. (1968). The phosphoric acid and phosphates used in food, pharmaceuticals and in the processing of foods and pharmaceuticals must, within stringent limits, be substantially free from arsenic, fluorides, heavy metals and general contaminants referred to as insolubles.
The thermal process for producing these highly pure food and pharmaceutical grade phosphoric acid provides extremely pure phosphoric acid, but requires extensive capital investment for equipment, together with a large input of energy and pollution control equipment. Until recently, the inability to confidently and consistently remove contaminants made electrothermal phosphoric acid prepared from elemental phosphorous the predominant source of such high purity phosphoric acid.
The so-called wet process employing phosphate rock and sulfuric acid is well known in the art and is described in Waggamen, Phosphoric Acid, Phosphates and Phosphatic Fertilizer, pp. 174-209, Hafner Publishing Co. (2nd Ed., 1969). The wet process is also broad enough to include acids other than sulfuric, including nitric and hydrochloric acid. This is described in Slack, Phosphoric Acid, V. 1, Part 2, pp. 889-926, Marcel Dekker, Inc. (1968).
The wet process produces a reasonably pure phosphoric acid, but one containing many contaminants that made it most suitable for such end uses as fertilizer and the like.
Many processes were proposed to remove the contaminants found in wet process phosphoric acid and substantially all the successful processes employ activated carbon. For example, see U.S. Pat. No. 3,993,733; U.S. Pat. No. 3,872,215; U.S. Pat. No. 3,993,736; U.S. Pat. No. 3,122,415; and British Pat. No. 1,442,919. Unfortunately, the phosphate rock impurities are found to deactivate the porous activated carbon and because sufficient of these impurities are inorganic, thermal regeneration of the activated carbon is unsuitable because phosphate glasses fuse to the pores of the activated carbon. Thus, the inability to suitably regenerate the activated carbon precluded its adoption in a commercial process on a cost competitive basis. Since the use of activated carbon was the key to so many successful processes for upgrading wet process phosphoric acid, extending the capacity of the activated carbon is a clearly desirable goal.
Improved regeneration techniques have increased the total number of useful cycles obtainable from a batch of activated carbon when used in the purification of wet process brown phosphoric acid. However, the total number of useful cycles available in no way increases the capacity of the activated carbon. Increase of capacity is especially important since as capacity increases, plant capacity increases, thereby decreasing capital costs per unit of output. Furthermore, the frequency of regeneration can be decreased with increased decolorizing capacity thereby saving down time and other non-productive operating costs. This extension results in less down time and more acid processed per unit of activated carbon.
Implementing the process of this invention results in helping to make wet process phosphoric acid the equivalent of thermal elemental phosphoric acid, and in making processes using activated carbon into commercially feasible ventures.